MDBG is a Chinese dictionary with plenty of features. It stays useful throughout your learning cycle, from beginner with Pinyin and pronunciation to expert with all sorts of additional information about characters, variants and more. A fantastic tool and a must for every learner of Mandarin Chinese.
Anki is a great free flashcard tool that's available for desktop, online and mobile. It comes with a list of flashcard decks ready for you to download but also allows you to build your own ones if you wish. Anki has some great HSK decks that are ideal for people starting out with learning Chinese. Some of them are still old HSK, but those are still useful if your main goal is to build up useful vocabulary quickly. If your goal is to build up vocabulary, make sure you also take a look at our own learning resources.
Youtube is a logical one and has many of the popular shows like e.g. 我是歌手, but is only really useful if you're not in Mainland China. For more advanced learners, make sure you check out Youku and Tudou. Both are to some extent similar to Youtube and contain loads of videos in Mandarin Chinese. Regardless of where your interests are, you'll for sure find something on there.
Wikipedia for Mandarin is ok, but might not be as good a resource for learning Mandarin as it is for other languages, especially if you concentrate on simplified characters. Baidu Zhidao is kind of similar to Wikipedia, but also different. Nevertheless, it's a great tool for learning Mandarin in that it has forum-like entries for a lot of topics, almost anything you can think of. For advanced learners it's particularly useful to check out differences between certain words or the history behind some of the 成语s.
Baidu is basically the Chinese Google and in general is extremely useful for learning Mandarin. However, they do also have loads of additional applications that might be lesser known but are highly useful, depending on what learning type you are. Baidu Wenku and Baidu Yuedu are great ways to find reading resources in fields you're interested in. Baidu Baike and Baidu Fanyi can be of great help in trying to make sense of certain Chinese words and phrases. For gamers, Baidu Youxi might be a great way to learn Chinese through games. These are only a couple of examples, here's the full list of Baidu apps for you to look through.
This is a tricky one, as it's hard to say if that's free or not, but depending on your background, it can be. I myself did an internship with InternChina quite some time ago in Qingdao and it has been one of the biggest boosts in terms of language acquisition I had while studying Mandarin. There are also other providers out there and opportunities for teaching English in China, though that's not as easy anymore as it used to be. It's hard to give too much guidance here, as the ideal thing will vary quite a bit depending on what your background is and what you want to achieve, but having some sort of longer stay inside Mainland China is definitely something that will boost your Mandarin skills up a couple of levels.
If you're outside of Mainland China, either Meetup.com or a visit to your local university are great ways to find groups or individuals that want to help you learn Chinese in exchange for you teaching them English or whichever other language you know. In Mainland China, things might be even easier. Again, universities are great places to look for people who want to brush up their own English and are willing to help you with your Mandarin, but in general, you'll have plenty of opportunity to find language exchange partners wherever you go, so there's no need to spend too much on this here I think.
Long gone are the times when you had to pay for your newspapers, most of them are available for free now online and news in China is no exception. Xinhua, Baidu News, QQ and Sina are only some of the vast list of news sites with articles from any topic you can think of. For more, go to Baidu and search for 新闻.
If you are outside of Mainland China, Youtube might actually be the best source with Chinese music for you. Within the Mainland, your choices are almost endless. Baidu Yinyue and QQ Yinyue are two great places to start. Also make sure you check your TV for any of the more popular music shows like 我是歌手, 我想跟你唱, or whatever might be broadcasting at the time you read this.
Shortly mentioned in point 9 already, TV is an endless resource of free Mandarin lessons. Chinese TV series, sport coverage, news, whatever you are interested in, there's likely at least one channel just for your area of interest. For watching TV directly it helps to be directly in Mainland China. Having said that, you might also be able to find some more popular shows on Youtube if you're outside of the Mainland - make sure you use characters when you make the search though.
Finally, Weibo. The ultimate resource for you to dive into Chinese culture. I put it last because the Mandarin used on "Chinese Twitter" isn't really the Mandarin you should use in your HSK exam, but I had to put it in as it's just an amazing source of information. Just get started with any topic or account you're interested in and slowly emerge yourself in content that's often a lot richer than what you're used to from "Western Twitter".
With the internet you have a bigger amount of information and resources at your fingertips than ever before. The vast amount of free resources online is often best utilized in conjunction with either language courses (online or in person) or other structured learning material like our Mandarin vocabulary resources. I hope this article can help you a bit to speed up your progress and make learning Chinese more interesting!